An Endospore is a variation of a bacteria cell that acts as a survival pod. It is a stronger, more resistant form of cell that can last for years without nutrients or moisture, and become reproductive when conditions improve.
Endospores are encased in a Spore Coat, which is made of protein, and acts as a shield against contaminants and poisons. Underneath that is a thick wall of Murein (sugar and amino acid) called the Cortex, which allows the core to survive without water and also adds physical protection. Inside that is another structure made of Murein, the Core Wall, which adds another layer of chemical protection. Finally, the Core of the cell contains DNA, Dipicolinic Acid, Calcium, and other cell structures, but is non-metabolically active; it does not produce any energy and so does not need to absorb nutrients, water, or energy to survive.
Endospores take about eight hours to form, and so do not provide protection against sudden, catastrophic failure of the host organism's biosphere. They are created as a last resort when the host organism detects that living conditions are becoming hazardous. When conditions improve, chemical actions inside the endospore reactivate, allowing reproduction. They are highly resiliant and can survive extreme cold and heat, although prolonged high temperatures will kill most endospores eventually. Since they are not technically living, most anti-bacterial or antibiotic treatments will have no effect.